Carbon nanotubes have the structure that is formed by cylindrically winding thin layer(s) of graphite crystals, i.e., planar or curved graphene sheet(s) formed by arranging six-membered rings of carbon molecules as hexagonal patterns. The diameter of carbon nanotubes is in the range of from a few nanometers to several tens of nanometers, and the length thereof is not less than tens of times to thousands of times as long as the diameter thereof. Such carbon nanotubes are classified into single-walled carbon nanotubes formed by cylindrically winding a substantially single graphene sheet, and multi-walled carbon nanotubes formed by cylindrically winding two or more of graphene sheets.
Single-walled carbon nanotubes have a small outer diameter and a large surface energy. Therefore, each of carbon nanotubes does not separately exist as a single carbon nanotube, so that a plurality of carbon nanotubes gather to form bundle(s) to be stabilized. Although multi-walled carbon nanotubes have properties, such as electrical conductivity, high elasticity and high strength, single-walled carbon nanotubes have different properties from those of multi-walled carbon nanotubes. For example, single-walled carbon nanotubes have such electric properties that they become metallic and semi-conductive, such mechanical properties that they are very tough and highly elastic, such thermal properties that they have thermal conductivity superior to diamond, and such absorption and occlusion properties that they absorb and occlude molecules. Due to such properties, it is expected that single-walled carbon nanotubes are applied to various technical fields, such as fields of hydrogen absorbing materials, antistatic agents, electrically conductive inks, field effect transistors, fuel-cell catalyst carriers, and anode materials of secondary cells.
Carbon nanotubes are generally produced by various methods, such as arc-discharge, laser vapor deposition, and thermal CVD (chemical vapor deposition) methods. In the arc-discharge method among these methods, a voltage is applied between carbon electrodes, which are spaced from each other by a few millimeters in an inert gas, to deposit carbon nanotubes on a cathode by arc-discharge (see, e.g., JP 2004-210555 A and JP 2006-16282 A), and it is possible to inexpensively produce carbon nanotubes having smaller structural defects than those produced by other methods. In the arc-discharge method, it is possible to selectively produce single-walled or multi-walled carbon nanotubes in accordance with the presence of catalytic metals filled in a carbon rod, and it is also possible to control the diameter and length of carbon nanotubes in accordance with the kind of catalytic metals.
However, in conventional methods for producing carbon nanotubes by arc-discharge, such as the methods disclosed in JP 2004-210555 A and JP 2006-16282 A, a graphite rod filled with catalytic metals is used as an anode. For that reason, large amounts of amorphous carbon, nanoparticles (particles having a size of a few nanometers) of catalytic metals, graphite sputtered from the graphite rod are contained in crude soot obtained immediately after being synthesized by arc-discharge, so that there is a problem in that the purity of synthesized carbon nanotubes is low. Among these impurities, amorphous carbon can be easily removed by burning and oxidizing the crude soot, and the catalytic metals can be easily removed by treating the crude soot with an acid. However, in order to remove graphite from the crude soot, it is required to carry out centrifugal separation after a surface active agent is added to the crude soot, and it is also required to carry out a size exclusion chromatography, so that it takes a great deal of time. In addition, there is a problem in that defects are applied to the structure of carbon nanotubes by carrying out such purifying steps.